When exposed to the atmosphere, molten aluminum is easily oxidized and forms inclusions, such as large amounts of oxides. Examples of the inclusions include oxides, such as Al2O3, MgO, MgAl2O4, SiO2, silicates, Al.Si.O, FeO, and Fe2O3, carbides (Al4C3, Al4O4C, graphite carbon), borides (AlB2, AlB12, TiB2, VB2), Al3Ti, Al3Zr, CaSO4, AlN, and various halides.
Since the free energy of formation of a Mg oxide is lower than the free energy of formation of an Al oxide, it is believed that Mg of a molten aluminum-magnesium alloy (an aluminum-magnesium alloy is hereinafter also referred to as an Al—Mg alloy) is preferentially oxidized and forms MgO (magnesia) or Al2O3—MgO (spinel). Because of the high wettability of the oxides to a molten Al—Mg alloy (hereinafter also referred to as a molten metal), the oxides are present in the molten metal as precipitated or suspended inclusions.
These inclusions in a molten metal finally become non-metallic inclusions, which impair the quality of products, such as wrought materials, forged products, and die-casting products.
Thus, an in-line treatment, such as a treatment of a molten metal in a furnace using a gas or flux, filtration, or a rotating nozzle treatment, may be performed to remove inclusions from a molten metal in a melting furnace or a holding furnace or in another production step.
In a process for transferring a molten metal from a treatment bath to a casting mold after the treatment and a process for casting the molten metal in the casting mold, however, the molten metal is exposed to the atmosphere, and an oxide is formed on the molten metal surface.
Thus, in order to prevent Mg of an Al—Mg alloy from being oxidized in a molten metal, a few parts per million of beryllium (Be) is generally added to the Al—Mg alloy. Such a treatment suppresses the formation of MgO and Al2O3—MgO (Non-patent Literature 1).
However, workers who continuously inhale a fine powder or fumes of Be may suffer from chronic respiratory dysfunction. The addition of Be must therefore be suppressed in consideration of the health of workers and work environment.
With increasing awareness of recycling from the perspective of energy conservation and low environmental load, some Al—Mg alloys are manufactured by using raw materials containing a given amount of P derived from aluminum scrap. Thus, there is a demand for a technique for suppressing molten metal oxidation even when such raw materials containing a given amount of P are used.
A method for suppressing the molten metal oxidation of Mg in an Al—Mg alloy without the addition of Be is proposed in Patent Literature 1. More specifically, oxygen supply from Bi to Mg is suppressed by lowering the bismuth (Bi) content of an Al—Mg alloy to 30 ppm (0.003 mass %) or less and thereby reducing the amount of Bi on a molten metal surface. Furthermore, the molten metal surface is covered with an Al or Mg oxide film having a low oxygen diffusion velocity in order to suppress the formation of MgO in the molten metal.